The development of photovoltaic devices such as solar cells is important for providing a renewable source of energy and many other uses. The demand for power is ever-rising as the human population increases. In many geographic areas, solar cells may be the only way to meet the demand for power. The total energy from solar light impinging on the earth for one hour is about 4×1020 joules. It has been estimated that one hour of total solar energy is as much energy as is used worldwide for an entire year. Thus, billions of square meters of efficient solar cell devices will be needed.
Photovoltaic devices are made by a variety of processes in which layers of semiconducting material are created on a substrate. Layers of additional materials are used to protect the photovoltaic semiconductor layers and to conduct electrical energy out of the device. Thus, the usefulness of an optoelectronic or solar cell product is in general limited by the nature and quality of the photovoltaic layers.
One way to produce a solar cell product involves depositing a thin, light-absorbing, solid layer of the material copper indium gallium diselenide, known as “CIGS,” on a substrate. A solar cell having a thin film CIGS layer can provide low to moderate efficiency for conversion of sunlight to electricity. The CIGS layer can be made by processing at relatively high temperatures several elemental sources containing the atoms needed for CIGS. In general, CIGS materials are complex, having many possible solid phases.
For example, some methods for solar cells are disclosed in U.S. Pat. Nos. 5,441,897, 5,976,614, 6,518,086, 5,436,204, 5,981,868, 7,179,677, 7,259,322, U.S. Patent Publication No. 2009/0280598, and PCT International Application Publication Nos. WO2008057119 and WO2008063190.
The CIGS elemental sources must be formed or deposited, either individually or as a mixture, in a thin, uniform layer on the substrate. For example, deposition of the CIGS sources can be done as a co-deposition, or as a multistep deposition. The difficulties with these approaches include lack of uniformity of the CIGS layers, such as the appearance of different solid phases, imperfections in crystalline particles, voids, cracks, and other defects in the layers.
A significant problem is the inability in general to precisely control the stoichiometric ratios of the metal atoms in the layers. Many semiconductor and optoelectronic applications are highly dependent on the ratios of certain metal atoms in the material. Without direct control over those stoichiometric ratios, processes to make semiconductor and optoelectronic materials are often less efficient and less successful in achieving desired compositions and properties. For example, no molecule is currently known that can be used alone, without other compounds, to readily prepare a layer from which CIGS materials of any arbitrary stoichiometry can be made. Compounds or compositions that can fulfill this goal have long been needed.
A further difficulty is the need to heat the substrate to high temperatures to finish the film. This can cause unwanted defects due to rapid chemical or physical transformation of the layers. High temperatures may also limit the nature of the substrate that can be used. For example, it is desirable to make thin film photovoltaic layers on a flexible substrate such as a polymer or plastic that can be formed into a roll for processing and installation on a building or outdoor structure. Polymer substrates may not be compatible with the high temperatures needed to process the semiconductor layers. Preparing thin film photovoltaic layers on a flexible substrate is an important goal for providing renewable solar energy and developing new generations of electro-optical products.
Moreover, methods for large scale manufacturing of CIGS and related thin film solar cells can be difficult because of the chemical processes involved. In general, large scale processes for solar cells are unpredictable because of the difficulty in controlling numerous chemical and physical parameters involved in forming an absorber layer of suitable quality on a substrate, as well as forming the other layers required to make an efficient solar cell and provide electrical conductivity.
What is needed are compounds, compositions and processes to produce materials for photovoltaic layers, especially thin film layers for solar cell devices and other products.